Study On Temperature Distribution Of Drilling Bore In Air Drilling

The distribution of the borehole temperature plays a very important role during air drilling process, which has influence on multiple key factors, such as the distribution of the borehole pressure, the rheological behaviour of the drilling fluid, the frictional resistance and the borehole stability. For a safer and faster drilling, it’s crucial to understand the rule of the borehole temperature distribution during air drilling.However, the present computational models about the distribution of the borehole temperature during air drilling have the following problems:neglect of the influence by the major thermal effects including the rock breaking, the flow friction heat and the heat produced by the friction between the drill string and the borehole; apply the overall heat transfer coefficient as the local convective heat transfer coefficient.In order to get a more accurate temperature computational model of air drilling, the following researches were carried out by this study:1. According to the theory of heat transfer and the principle of energy conservation, we established a comprehensive heat transfer model composed of the stratum-the annular space-the drill string during air drilling.2. Through a series of comprehensive analyses on the rock breaking, the flow friction heat and the friction heat between the drill string and the borehole, we got the heat distribution ratio during the process of the rock breaking, the quantity of heat brought by gas-solid two-phase flow within the annular space during the friction between the drill string and the borehole and the heat produced by the flow friction.3. With the convective heat transfer coefficient of pure air across the annular space as standard, we applied three representative models to the actual operation of drilling. By considering the influence of rotation of the drill string, we selected the most suitable one for air drilling process.4. Based on the analyses of the borehole heat sources and convective heat transfer coefficient, we wrote the computer program and made the distribution curves of borehole temperature under three cases, analysed the influence caused by heat sources combined with the key points in each curve.In this study, we made a systematic analysis on the heat sources, optimized each parameter of the computational model, considered the influence of multiple heat sources and made the results more suitable for air drilling.